Phase Relationship and Lithium Deintercalation in Lithium Nickel Oxides

The phase relationship between LiNiO2 and its related compounds was examined by X-ray diffraction, neutron diffraction, DTA-TG, and electrical, magnetic, and electrochemical measurements. Decomposition of LiNiO2 proceeded at higher temperatures to Li1-xNi1+xO2 with the partially disordered α-NaFeO2 structure and LixNi1-xO with the fully disordered rock-salt structure. Higher oxygen partial pressure in the atmosphere suppressed the decomposition reaction, which was considered to be the reduction from Ni3+ to Ni2+ state. The nearly stoichiometric "LiNiO2" was synthesized at reaction temperatures below 700°C in an oxygen gas flow with the starting materials Li2O2 and NiO. Neutron diffraction measurement confirmed the composition Li0.996Ni1.006O2, which is very close to the stoichiometry. The deintercalation mechanism of LiNiO2 and the spinel transformation process from Li1-xNiO2 to LiyNi2O4 were examined; the deintercalation proceeded from LiNiO2 In Li0.3NiO2 monophasically when the cell configuration allowed a homogeneous reaction. The range of solid solutions 0.8 ≤ y ≤ 1.8 was observed for the spinel LiyNi2O4 transformed from Li1-xNiO2 (0.1 ≤ x ≤ 0.6) by heat treatment at 180°C. The room. temperature intercalation to the spinel LiNi2O4 gave the new polymorphism Li2Ni2O4 (LiNiO2) with the cubic lattice parameter a = 8.207(3) Å. Electrical properties of the spinels were also examined.